Collimation

The alignment of the optical elements of a telescope at the correct
angles to the light path. If not properly collimated, a telescope will
deliver distorted images ( lopsided or elongated stars, hazy planetary
images, an inability to split close binary stars, etc.) Atmospheric
turbulence ( seeing ), thermal currents within the telescope, and dirty
or defective eyepieces can often mimic poor collimation. The
illustrations below simulate the Airy disks and diffraction rings of a
properly collimated scope as well as those of a mis-aligned scope.